Many ways exist in the prior art for forming various hollow three dimensional shapes required for such purposes as mufflers for internal combustion engines, exhaust expansion chambers for internal combustion engines, and other such hollow three dimensional objects. In many cases the shapes required are complex and require different conical curved and tapered sections. For instance in the case of an expansion chamber such shapes are required for proper functioning of an engine with attenuation of the sound produced. Among the known methods of forming such devices are fabrication from individual pieces of sheet metal which have been given the required shapes in various forming processes but the method which is believed most similar to ours is the application of hydraulic pressure to a body which is enclosed in a cavity which represents the final shape to be achieved, usually with the edges of the sheet clamped between metallic surfaces so that the sheet must be stretched to a thinner gauge as it expands to the shape of the cavity it occupies.